The production of corn syrup and corn syrup solids, by the hydrolysis of starch, has progressed in the direction of ever sweeter products. The acid hydrolyzates that were the initial commercial products have gradually given way to generally superior products produced by the use of sacchrifying enzymes. The art has advanced to the point were enzyme hydrolyzates can be routinely produced on a commercial scale at D.E. values in excess of 95.
However, the industry is interested in even greater sweetness, and research has been conducted for years into techniques for isomerizing starch hydrolyzates to increase the content of levulose. Important initial work in the field was conducted by Cantor and Hobbs, as reported in U.S. Pat. No. 2,354,664, granted Aug. 11, 1944. Cantor and Hobbs utilized alkali catalysis to effect isomerization.
Alkali isomerization is limited in the degree to which the isomerization can be carried out efficiently and with the formation of commercially acceptable products. Consequently, there was a widespread search for many years for an enzyme that would effect the isomerization. This search culminated in the discovery that xylose isomerase, which catalyzes the interconversion of D-xylose and D-xylulose, did convert D-glucose (dextrose) to D-fructose (levulose), as described by Marshall and Kooi in Science, Apr. 5, 1957, Vol. 125, No. 3249, pages 648-649, and in the pioneer patent in the field, U.S. 2,950,228, to Richard O. Marshall, on Aug. 23, 1960. Since that time, there has been a great amount of research activity in connection with enzymatic isomerization.
The use of a microorganism of the Actinomycetales order, for the production of an isomerizing enzyme, was reported by Sato and Tsumura in their paper, "A Study on Isomerization of Dextrose by a Streptomyces Strain", at the Annual Meeting of the Agricultural Chemical Society of Japan held at Sapporo in July, 1964. A great deal of subsequent work, relating to the use of microorganisms of the Streptomyces genus for isomerase enzyme production, has been conducted at the Fermentation Research Institute of Japan, as reported by Dr. Y. Takasaki and his associates. Some of this work has been summarized in the publication, Fermentation Advances, Academic Press, New York, 1969, in the article by Dr. Takasaki et al. beginning at page 561.
The work of Sato and Tsumura led to the use of Streptomyces microorganisms for the production of isomerizing enzymes by the use of a nutrient medium containing xylose. Unfortunately, if xylose is required for enzyme production, there are limitations on the nature and cost of the medium that is required. Dr. Takasaki and his associates identified certain strains of Streptomyces that secreted xylanase, and that therefore could be cultured in nutrient media containing xylan, which is much less expensive than xylose. Unfortunately, the economics and other limitations on the nature and cost of the culture media required even for these microorganisms impose severe limitations on the process. Moreover, all known strains have been thought to require the presence of cobalt in the culture medium for practical enzyme production, and this created a disposal problem.